I. Technical Field
The present invention relates to a method of manufacturing a chromium plated article and a chromium plating apparatus, which can deposit a hard chromium layer on a work surface.
II. Description of the Related Art
In a hard chromium plating treatment for a general purpose, a hard chromium layer is deposited on a work surface. There are disadvantages in the method, in that cracks may appear in the chromium layer which reach a metal substrate, allowing substances which cause corrosion to come in contact with the metal substrate, thereby deteriorating the ability of the plated article to resist corrosion. Therefore, it has hitherto been general practice that selected parts of articles which are exposed to corrosive environments are subjected to a pre-treatment procedure of nickel plating or copper plating so as to form an undercoat having a film thickness of the same extent as the layer of hard chromium plating that is applied subsequently. However, this requires that the plating treatment be carried out twice by using different procedures for a pre-treatment step to create an undercoat and the subsequent step of applying a layer of chromium plating, with the result that there is an increase in manufacturing cost.
On the other hand, it has already been ascertained that a chromium layer free from cracks can be formed by performing what is called pulse plating by use of a pulse current (refer to the Japanese Patent Application Disclosure No. 3-207884 (U.K. Patent Application Disclosure No. 2,236,763), for example). When this method is adopted, it becomes possible to obtain in one treatment step chromium plated parts with excellent corrosion resistance. This chromium treatment which is performed by using a pulse current has the problem that when parts are subjected to a thermal history, large cracks are apt to occur in a chromium layer. Therefore, there has been no other choice than to abandon the application of this chromium treatment to parts subjected to a thermal history.
It might be thought that a tensile stress generated in a chromium layer due to thermal contraction is responsible for the phenomenon that cracks are apt to occur when parts are subjected to a thermal history. It follows, therefore, that the occurrence of cracks can be suppressed by causing a compressive residual stress capable of counterbalancing the above-described tensile stress to be present beforehand in the chromium layer or by suppressing the quantity of thermal contraction of the chromium layer itself. In this case, the thermal contraction of the chromium layer is affected by the quantity of lattice defects which are present in large amounts at the grain boundary of the chromium layer and, therefore, the total amount of the boundary is reduced by increasing the size of crystallites of the chromium layer (the total amount of the boundary is inversely proportional to the size of crystallites), whereby lattice defects are reduced and thermal contraction of the chromium layer can be suppressed.
Paying attention to the above-described point, Yuichi KOBAYASHI, who is one of the present inventors, et al. have ascertained that the above-described occurrence of cracks caused by a thermal history can be prevented by causing a compressive residual stress of not less than 100 MPa to be present in the chromium layer and ensuring the size of crystallites of the chromium layer which is not less than 9 nm on average (however, the upper limit being 16 nm on average), and have already made this fact clear in the Japanese Patent Application Disclosure No. 2000-199095 (U.S. Pat. No. 6,329,071). And Yuichi KOBAYASHI et al. found out that as a method of depositing a chromium layer having such a large compressive residual stress and size of crystallites, it is effective to perform electroplating while adjusting the pulse waveform of a pulse current in a plating bath containing an organic sulfonic acid, and have also made this point clear in the Japanese Patent Application Disclosure No. 2000-199095.